1. Field of the Invention
The present invention relates to (i) a calculation method and an apparatus for obtaining at least one of information regarding a refractive index and a thickness of a specimen by utilizing the fact that a propagation state of an irradiated electromagnetic wave changes due to the refractive index or a dielectric constant of the specimen, (ii) a material for calculating a refractive index, and (iii) a tomography apparatus.
2. Description of the Related Art
Calculation of the refractive index has been used widely in an optical band, and the calculation is also considered recently to be important for electromagnetic waves in a frequency band of a millimeter wave and a terahertz wave (30 GHz or higher and 30 THz or lower) (hereafter also referred to simply as a terahertz wave). This is because physical properties such as a refractive index and a dielectric constant of a specimen in this frequency band are different from those in the optical band, and that these characteristic properties can be used for identifying physical properties in a tomography apparatus, or the like. For instance, many biomolecules and pharmaceuticals have characteristic absorption spectra in this frequency band. As methods of calculating the refractive index in this frequency band, the following methods have been typically used.
As one of the methods, there is known a method of calculating the refractive index by irradiating a specimen with an electromagnetic wave in this frequency band, and comparing a waveform of a reflected wave thereof with the original waveform, for example, in a case without the specimen (see Japanese Patent Application Laid-Open No. H11-108845). According to this method, it is possible to map the refractive index in a depth direction of the specimen. Therefore, the method is one of important algorithms for forming a tomogram.
In addition, as a base of this algorithm, the original waveform to be compared with the waveform of the reflected wave is important. A waveform of an incident wave is used in the above-mentioned example, but various variations can be considered, depending on an optical system and the specimen. For instance, there is known a method in which a Fresnel reflection waveform on a surface of an attenuated total reflection (ATR) prism used in an optical system, called an attenuated total reflection spectroscopy, is used as the original waveform to be compared (see D. Woolard, W. Loerop, and M. Shur, translated by Omori and Hirose, “Terahertz Sensing Technology, vol. 1”, NTS publication (2006) chapter eight, page 289). In the optical system of the ATR spectroscopy, in which the ATR prism and the specimen contact closely with each other, it is possible to eliminate an influence of a refractive index of the ATR prism. Therefore, it is possible to obtain the refractive index of the specimen at high sensitivity.
In the frequency band of the millimeter wave and the terahertz wave (30 GHz or higher and 30 THz or lower), absorption by water, however, cannot be neglected. Therefore, the algorithm of Japanese Patent Application Laid-Open No. H11-108845, in which an approximation of neglecting electromagnetic wave absorption is used, is difficult to be applied to a specimen, such as a biomolecule, having a relatively large electromagnetic wave absorption. This is because Fresnel loss and absorption loss cannot be distinguished between the waveform of the reflected wave and the original waveform to be compared. Therefore, depending on the specimen, it is difficult for the conventional method to calculate a refractive index distribution or a dielectric constant distribution in the depth direction of the specimen.